1. Field of the Invention
The present invention relates to an input interface circuit disposed in the input stage of a semiconductor integrated circuit.
2. Description of the Related Art
Signals to be input to an input interface circuit are not always in full swing from the power supply potential VDD to the ground potential GND, but may have a high level potential which is lower than the power supply potential VDD. Therefore generally an input interface is designed so as to operate even if the high level potential of the input signal is lower than the power supply potential VDD. For example, the input interface circuit may be designed assuming that the power supply potential VDD is 5 volts and the high level potential of input signals is 2.4 volts.
Normally in an input interface circuit, the threshold value to determine the high level/low level of input signals is set to a mid-value between the low level potential and high level potential. When the high level potential is 2.4 volts, for example, the threshold value is around 1.2-1.6 volts. In other words, in order to create an input interface circuit which power supply potential VDD is 5 volts and the high level potential of input signals is 2.4 volts, each transistor in the input interface circuit is designed such that the threshold value becomes 1.2-1.6V when VDD is 5 volts.
Because of this, the input interface circuit cannot insure normal operation if the power supply potential VDD which is actually used is different from the power supply potential VDD which was assumed during design.
For example, in the case of an input interface circuit where the threshold value becomes 1.2 volts when VDD is 5 volts, the threshold value when VDD is 3 volts becomes lower than 1.2 volts. Therefore the possibility of erroneously recognizing a low level signal as a high level signal increases due to signal noises.
If a 5 volt VDD is supplied to an interface circuit where VDD is designed as 3 volts, on the other hand, the possibility of erroneously recognizing a high level signal as a low level signal increases.
Therefore, only one type of power supply voltage can be used for a conventional interface circuit.
An object of the present invention is to provide an input interface circuit which can be used with a plurality of types of power supply voltage.
An input interface circuit in accordance with the present invention comprises: means for comparing the potential level of a signal input from a signal input terminal with a predetermined threshold value and outputting the comparison result from a signal output terminal; means for detecting the power supply potential to be supplied to the comparison means; and means for controlling the threshold value of the comparison means according to the power supply potential detected by the detection means.
The input interface circuit of the present invention can change the threshold value of the comparison means according to the power supply potential, so a plurality of types of power supply voltage can be used.